Use of Qt standard icons from within Qt Creator

Theme icons are only supported on Linux/X11. On Windows and OS X, you have to provide your own icons.

The solution to this problem is to set the theme name in QtCreator (as in your example). Then you have to provide icons of the same Alias under Prefix ":/icons" in your resources. You can leverage icons from sets available in the public domain, e.g. the popular Tango icon set.

If you only target the Windows platform, the theme setting will be of no use for you. If you target both Linux/X11 and other platforms, with this solution you get the native icons on one system and the icons you provide yourself on the others.

You can find a very good explanation on how to do it here: http://mithatkonar.com/wiki/doku.php/qt/icons

@ypnos suggested a great link with multiple approaches to solve the problem. I favorite the last one - Creating a custom icon theme. The author states three Pros (Available to all platforms, Excellent resizing, Covers all needed icons), and two Cons (Not consistent with system icon theme, Pain in the ass to implement). Here I suggest improvements to get rid of the Cons.

I've cloned the Tango iconset from github. Repository https://github.com/ppinard/qtango already features the index.theme file. But mainly, it brings a Python script generate_rcc.py, which can generate the *.qrc file automatically. I just had to change the arguments of subprocess.check_call() from '--binary' and '--compress' to '-binary' and '-compress'. The generated file contained absolute paths, but that's easy to Find&Replace. One can use this script to any iconset - first laborious step is solved.

Now, using the "Theme" property, you can define the icons in the Qt Designer, as already shown in the question. For those developing under Linux, system icons will show right in the Designer (suppose the iconset uses standard icon names). That's the native look (icons will be configurable in your system settings). That frees you from the ui->action_Open->setIcon(...) coding.

And the final tweak is to set the theme before ui setup.

MainWindow::MainWindow(QWidget *parent) :
  QMainWindow(parent)
{
#ifdef _WIN32
  QIcon::setThemeName("tango");
#endif

  ui = new Ui::MainWindow;
  ui->setupUi(this);

  ...
  // NO NEED FOR ui->action_Open->setIcon(...)
}

The #ifdef can of course be adjusted to target all needed platforms, or omitted to force the same icons on all platforms including Linux.

As a result, this approach avoids all laborious coding and the result is consistent with the system icons at least on Linux.